The present invention relates to ophthalmic lenses, for example, to ophthalmic lenses for short-sightedness, the power of which is between -5 and -20 dioptres.
As is known, the power of a lens is defined in general terms by the curvature given to its front and/or back faces, the front face being conventionally designated as that face which is furthest from the eye to be corrected, and the back face being conventionally designated as that face which is closest to this eye. In practice, the power of an ophthalmic lens is equal to the sum of the powers of its faces within the limits of the thickness correction.
As is also known, such faces are most frequently surfaces of revolution about the optical axis of the lens; for certain corrections, however, one or both of these faces can be toric or cylindrical or can even have a radius of curvature which can vary according to a given particular law.
In general terms, ophthalmic lenses for short-sightedness, having a power of less than -5, belong to one or other of three different types, which are as follows:
Firstly, there are plano-concave lenses, that is to say lenses of which the front face is substantially planar or slightly convex, this front face having only a low power and all the power being provided by the back face of such a lens.
Because of this power contrast, the front and back faces of such a lens rapidly diverge from one another with the result that, at its periphery, this lens possesses a rim of which the thickness, measured axially, that is to say parallel to its axis, is large.
Such a rim has an unfavourable influence on the weight of the lens at the expense of comfort to the user and makes the lens unattractive.
Furthermore, with such a lens, there is a practical limit to the aperture diameter which it is possible to give to the lens for high powers, it being necessary for the back face to have a virtually hemispherical shape as soon as the power is less than -15 dioptres.
In order to reduce the weight of the lens, it is customary to reduce the axial thickness of its peripheral rim outside that which is considered to be the useful zone of such a lens.
For example, an annular surface, which is either planar or convex or formed firstly of a convex part and then of a planar part, is formed outside this useful zone.
In all cases, this results in the formation of angular edges, imparting bull's eye effects to the lens, which is unattractive.
Then there are bi-concave lenses, that is to say lenses of which both the front and back faces are concave and possess substantially the same power.
However, in this case, the aberrations due to each of these faces are cumulative, which is unfavourable; moreover, it is most frequently necessary, as above, to reduce the thickness, and thus the weight, of the peripheral rim of the lens, and this also leads to a bull's eye effect.
Finally, there are concavo-convex lenses, referred to as meniscus lenses, that is to say lenses of which the front face is distinctly convex, whilst at the same time possessing a lower power than that of the back face, the latter being concave; this type of lens is described, in particular, in U.S. Pat. No. 949,501.
Such lenses advantageously possess, at their periphery, a rim of relatively reduced thickness.
Moreover, they have relatively good optical properties, in particular low distortion and acceptable astigmatism.
However, they are unable to make allowances for other defects, in particular the curvature of field.
In fact, it is shown that any action taken on the astigmatism can be to the detriment of the curvature of field.
In other words, an ophthalmic lens can be good from the point of view of the astigmatism and be poor from the point of view of the curvature of field.
It is an object of the present invention to provide an ophthalmic lens which, even when of high power, possesses a peripheral rim having a relatively reduced thickness, whilst at the same time possesses the best possible optical properties, which are at least as good as, or even better than, those of the comparable optical lenses known hitherto.